1. Field of the Invention
The present invention relates to a detector for detecting an air-fuel ratio, or a ratio of air to fuel in an internal combustion engine or the like, and a method of measuring an air-fuel ratio.
2. Description of the Prior Art
It is currently known to detect oxygen density in an exhaust gas emitted from an internal combustion engine of an automobile or the like and control the amounts of air and fuel to be supplied to the internal combustion engine based on the detected oxygen density value, thereby reducing harmful components in the exhaust gas.
Air-fuel ratio detectors (oxygen sensors) presently available for internal combustion engines for automobiles for detecting oxygen densities operate on the principle of an oxygen concentration cell. This type of air-fuel ratio detector is capable of detecting a stoichiometric air-fuel ratio (A/F=14.6) because of its characteristics. However, it cannot detect air-fuel ratios in other ranges, that is, a lean range in which the air-fuel ratio is higher than the stoichiometric air-fuel ratio or a rich range in which the air-fuel ratio is lower than the stoichiometric air-fuel ratio. When a voltage is applied between gas-permeable thin-film electrodes attached to a solid electrolyte cell permeable to oxygen ions, oxygen ions pass through the cell from the cathode to the anode, and with the oxygen ions flows an electric current between the electrodes. If the quantity of oxygen ions which are going to pass through the cell is limited as by forming a porous ceramic coating layer on the cathode, then limited-current characteristics appear in which the current is not increased beyond a certain value even when an applied voltage is increased. Utilizing such phenomenon, there has been developed a limited-current oxygen sensor for detecting oxygen densities with a view to detecting air-fuel ratios in the lean range. Since the limited-current oxygen sensor is capable of detecting air-fuel ratios in the lean range only, such is called a "lean sensor", and is almost incapable of detecting air-fuel ratios in the rich range.
While an automobile is running under ordinary conditions, it is preferable to be driven in the lean range in which an air-fuel mixture is lean. When the engine is required to produce a higher power output such as during travel up a slope, the automobile is preferably driven in the rich range. Therefore, there has been a demand for a detector capable of detecting an air-fuel ratio in the rich to the lean range.